1. Field of the Invention
This invention relates to the field of electrical switches and particularly to the field of remotely controlled electrical switches capable of interrupting the application of a source voltage to a load in response to an overload condition.
This invention relates more particularly to the field of SSPC (solid state power controllers) for ac (alternating current) service or solid state electrical switches such as solid state relays for use in aircraft applications capable of detecting overload conditions, such as those caused by battle damage. Solid state power controller ac switches are able to interrupt electrical service to the load automatically thereby preventing or limiting further damage to the service or load or lead wire. This invention off-line capacitor divider supply circuit has the capability of supplying several voltage levels to the control and drive circuitry used within SSPCs. The invention circuit is line powered and referenced to the line voltage level, a fact that permits it to provide the required dc levels to circuitry also referenced to the line voltage level, i.e. typically 230 V ac, 400 Hz. The circuit provides a light weight, noise free, low cost solution having low power dissipation, immunity to high vibration, mechanical shock and high reliability.
2. Description of the Prior Art
Electromechanical switching devices using solenoid driven means to transfer electromechanical contacts to apply a source voltage to a load, such as relays are well known. When coupled with an electromechanical circuit breaker, a relay provides a remotely controllable power control function capable of supplying ac or dc service to load via its closed contacts in series with an electromechanical circuit breaker. Electromechanical switching devices, such as relays, provide a very low voltage drop at the switch closure thereby affording low power dissipation.
As used in ac power control applications, mechanical contacts cannot apply or remove power to a load free of bounce and arcing. Mechanical wear, electrical arcing and slow response also limit the application of electromechanical devices to applications requiring low closing cycle rates.
The SSPC is known to overcome the problem of contact bounce in applying a source voltage to a load. Solid state ac switches are also able to interrupt load current. However, SSPCs require low power dc voltage sources derived from the line service to operate their associated control and switch drive electronics. The electronic circuitry used within the SSPC is typically referenced to the ac power line voltage level and not to ground. Referencing the SSPC switch circuitry to the line voltage and not to ground or neutral permits all power to be removed from a serviced load as the SSPC switch is open. Control circuitry within tne SSPC remains active since the low voltage ac sources are referenced to the line voltage.
If the control circuitry within the SSPC is referenced to ground or neutral and the load is interposed between the ac line service and the switch, the load will be at line voltage as the SSPC switch is opened. Conventions of safety mitigate against this arrangement.
Typical prior art dc sources for use with SSPC's employ a power transformer and a rectifier and filter with the associated disadvantage of weight and size. As line voltage is increased, power transformers have the added disadvantage of requiring heavier insultion which adds to the size of the SSPC dc source. Efficient transformer design typically drives the geometry of the transformer to be cubic in character and not flat, which is the preferred geometry for circuit board construction.